The determination of the swelling-clay content, especially smectite content, of cement mixtures (also referred to as "cement mix") is important in the design and evaluation of cement slurry formulations for use in the exploration for and the production of hydrocarbons. The cement setting process is a chemical reaction, and the chemical environment, including the swelling-clay content of the cement slurry, will affect the behavior of the cement slurry and will affect the properties of the cured cement. Parameters such as wellbore fluids and reservoir characteristics such as formation types and formation fluids will influence the slurry design and operation during cementing.
Bentonite (a smectite clay, also commonly referred to as "gel" in oilfield terminology) is a swelling clay which is often added to cement mixtures to decrease the slurry density, but it can significantly reduce the cement strength if excessive amounts are used. Bentonite can also enter the cement slurry during drilling fluid displacement, and it is often desirable to analyze return cement from failed cement jobs to assess this possibility.
Drilling fluid removal is a critical step in the cementing process. A successful cement job requires that all the cement be bonded to the casing and to the formation. This bonding requires that the drilling fluid ("drilling mud" or just "mud") that previously occupied that space in the wellbore be completely displaced by special preflushes and/or the cement slurry. Failure to remove all the mud may result in channels of mud along the length of the pipe or borehole. Because mud has no bonding strength and is easily flocculated by formation fluids, channels result in a poor cement job. Also, channels within the set cement may allow undesired migration of well fluids into or out of the formation. Migration of hydrocarbons from a high pressure zone to other zones can result in economic loss. Also, a poor cement job can result in poor confinement of stimulation treatments. Acidizing or hydraulically fracturing a zone that is not isolated by a competent cement sheath can result in the loss of treating fluids into undesirable zones. Breaking out of the productive zone limits the penetration by the treating fluid and may irreparably destroy the natural confining barriers.
Also, failure to displace mud ahead of the cement may lead to severe contamination of the cement with swellable clay and drill solids. This contamination causes weakening of the cement. As mentioned earlier, severe contamination may also occur if excessive amounts of swelling clay (e.g. bentonite) are added to reduce the density of the slurry.
Contamination of cement by swellable clays, either from mud contamination or excess bentonite addition, results in poor quality set cement. The result can vary from production of unwanted fluids to attack on the casing by corrosive waters. A poor cement job may also result in insufficient cement strength to support casing which in turn may result in casing movement, loss of casing integrity, and inability to support well control equipment.
It is desirable, therefore, to be able to obtain, at the wellsite, timely estimates of the swelling-clay content of cement mixtures, both before the cement is used and in cured samples that may be obtained from the subsurface.
Two of the most common methods for determining the swelling-clay content of a sample are the X-ray diffraction method and the cation exchange capacity (CEC) method. Both of these methods are well established in the art. Briefly, the X-ray diffraction method provides semi-quantitative mineral contents. See for example, "Crystal Structures of Clay Minerals and their X-ray Identification", edited by G. W. Brindley and G. Brown, Mineralogical Society, 1980, London, Chapter 5, for a brief summary of the X-ray diffraction method used in determining swelling-clay content. The CEC method, on the other hand, correlates the number of exchangeable cations in a sample (cations in the sample that can be replaced by another cation such as barium or ammonium) to the swelling-clay content. See for example, Van Olphen, H., "An Introduction to Clay Colloid Chemistry", Wiley-Interscience, New York, 1977, Chapter 5, and American Petroleum Institute publication RP 13B, "Recomended Practice for Standard Procedure in Testing Drilling Fluids", for brief summaries of CEC methods used in determining swelling-clay content.
However, it is also well established that the X-ray diffraction method usually requires days to complete and has to be performed in the lab. Furthermore, the X-ray diffraction method requires a sedimented, oriented deposit of clay crystals for quantitative results. Swelling clay particles that are part of cured (set) cement will not orient properly during the sedimentation process. The CEC method requires significant care in running the test and is not particularly suitable for wellsite use. In addition, the CEC method will not work for samples of set cement because of interference by the cement and alteration of the clay. Thus, neither of these methods are suitable for measuring swelling-clay content of cement mixtures, especially when the cement is cured.
There exists a need, therefore, for a rapid and reliable wellsite method for the determination of the swelling-clay content in cement mixtures.
Most recently, U.S. Ser. No. 175,081 to Kroeger et al. discloses a method for determining at wellsites the swelling-clay content of shales and shaly sandstone earth formations by dielectric measurements. Kroeger et al.'s method includes washing a sample of an earth formation with a fluid having a water activity substantially less than that of water, which fluid may contain a soluble cation, measuring the sample's dielectric constant at a preselected frequency, and comparing the results of this measurement to calibration curves to obtain a measurement of the swelling-clay content of the formation. Kroeger et al.'s method describes different levels of determinations depending on the nature of the formation samples.
Dielectric measurements are also utilized for other, unrelated purposes. For example, dielectric measurements are utilized in logging tools for making determinations of the water and hydrocarbon content in sandstones and carbonates. These logging tools are not designed for making swelling-clay content determinations. In addition, these logging tools lose their effectiveness in high-salinity formations.
To the best of Applicants' knowledge, dielectric measurements are not used for making determinations of swelling-clay content of cement mixtures. In fact, excluding U.S. Ser. No. 175,081 to Kroeger et al., prior art actually dismisses dielectric responses observed between 1-50 MHz in dilute aqueous swelling-clay suspensions as anomalies which vanish with increasing salinity. See for example, Raythatha, R. and Sen, P. N., "Dielectric Properties of Clay Suspensions in the MHz to GHz Range", Journal of Colloid and Interface Science, Feb. 1986, Vol. 109, No. 2, in general, and particularly see pages 305 and 308 wherein it is stated that the electrochemical effects (of swelling-clays) become unimportant at high salinities and the geometrical effects dominate.